This invention relates to an apparatus and a method for image processing, particularly ones that are useful in reading the images on photographic films (which are hereinafter referred to simply as xe2x80x9cfilmsxe2x80x9d) to produce photographic prints of high quality in succession.
Heretofore, the images recorded on films such as negatives and reversals have been commonly printed on light-sensitive materials (photographic paper) by means of direct (analog) exposure in which the film image is projected onto the light-sensitive material to achieve its areal exposure.
A new technology has recently been introduced and this is a printer that relies upon digital exposure. Briefly, the image recorded on a film is read photoelectrically, converted to digital signals and subjected to various image processing operations to produce image data for recording purposes; recording light that has been modulated in accordance with the image data is used to scan and expose a light-sensitive material to record a latent image, which is subsequently developed to produce a finished print. The printer operating on this principle has been commercialized as a digital printer.
In the digital printer, images can be processed as digital image data to determine the exposure conditions for printing. Hence, it is capable of performing various image processing operations such as the correction of washed-out highlights or flat (dull) shadows due to the taking of pictures with backlight or an electronic flash, sharpening and the correction of color or density failures and this enables the production of prints of the high quality that has been impossible to achieve by the conventional direct exposure. Further, not only the assembling of images and the splitting of a single image into plural images but also the composition of characters can be performed by processing the image data and, as a result, prints can be outputted after various editing and/or processing operations have been performed in accordance with specific uses.
Outputting images as prints is not the sole capability of the digital printer; the image data can be supplied into a computer and the like and stored in recording media such as a floppy disk; hence, the image data can be put to various non-photographic uses.
Having these features, the digital printer is basically composed of an image input unit having a scanner (image reading apparatus) and an image processing apparatus, and an image output unit having a printer (image recording apparatus) and a processor (developer).
In the scanner, reading light issuing from a light source is allowed to be incident on a film, from which projected light bearing the image recorded on the film is produced and focused by an imaging lens to form a sharp image on an image sensor such as a CCD sensor; the image is then captured by photoelectric conversion and sent to the image processing apparatus as data for the image on the film (as image data signals) after being optionally subjected to various image processing schemes.
In the image processing apparatus, image processing conditions are set on the basis of the image data captured with the scanner and image processing as determined by the thus set conditions is performed on the captured image data and the resulting output image data for image recording (i.e., exposure conditions) are sent to the printer.
In the printer, if it is of a type that relies upon exposure by scanning with an optical beam, the latter is modulated in accordance with the image data from the image processing apparatus and deflected in a main scanning direction as the light-sensitive material is transported in an auxiliary scanning direction perpendicular to the main scanning direction, whereby a latent image is formed as the result of exposure (printing) of the light-sensitive material with the image bearing optical beam. Then, in the processor, development and other processing as determined by the light-sensitive material are performed to produce a print reproducing the image that was recorded on the film.
The image processing schemes described above generally include color balance adjustment, contrast correction (toning), lightness correction, dodging (compression/extension of the dynamic range of densities), chroma correction and sharpening. These schemes are performed by known methods comprising appropriate combinations of arithmetic operations with operation expressions, processing with look-up tables (LUTs), matrix (MTX) operations, processing with filters and the like. For example, color balance adjustment, lightness correction and contrast correction are performed with LUTs and chroma correction is performed by MTX operations. Sharpening and dodging are performed in other processing units.
In the conventional image processing technology, color signals are uniformly processed with a set of matrix coefficients. However, a problem with such uniform processing is that xe2x80x9cgrayxe2x80x9d signals vary subtly to cause variations in color tints. If the color signal space is divided into a plurality of regions which are to be processed with different matrices, several processing problems occur; first, the respective matrices have to be calculated; second, the boundaries of the individual regions must be connected smoothly; third, even gray has to be corrected with matrices.
The present invention has been accomplished under these circumstances and has as an object providing an image processing method in which secondary matrix operations are performed on the color reproduction area taken as a whole but in which the secondary matrix operations are not performed or LUT operations are performed on xe2x80x9cgrayxe2x80x9d taken as a separate entity, thereby ensuring that the desired color reproduction can be performed consistently without causing subtle variations in xe2x80x9cgrayxe2x80x9d signals. Another object of the present invention is to provide an image processing apparatus for implementing the above method. The stated object of the invention can be attained by an image processing method comprising the steps of: performing nonlinear transformation on a digital color signal to obtain a nonlinear transformed signal; computing a chroma of the digital color signal from the digital color signal or the nonlinear transformed signal; computing a chroma weight coefficient based on the computed chroma; and performing arithmetic operations for weighting the nonlinear transformed signal and the digital color signal with the chroma weight coefficient, thereby obtaining an output signal.
Preferably, the image processing method of the present invention further includes the steps of: computing a lightness of the digital color signal from the digital color signal or the nonlinear transformed signal; computing a lightness weight coefficient based on the computed lightness; combining the lightness weight coefficient and the chroma weight coefficient to compute a single coordinated weight coefficient; and performing arithmetic operations for weighting the nonlinear transformed signal and the digital color signal with the coordinated weight coefficient, thereby obtaining the output signal.
Preferably, in order to determine the coordinated weight coefficient, a sum or product of the two weight coefficients including the lightness weight coefficient and the chroma weight coefficient is computed and limiting is performed such that the coordinated weight coefficient takes a value within a specified range.
The nonlinear transformation is preferably a secondary matrix or a three-dimensional LUT.
The stated object of the invention can be attained by an image processing method comprising the steps of: performing nonlinear transformation on a digital color signal to obtain a nonlinear transformed signal; performing LUT transformation on the digital color signal or the nonlinear transformed signal to obtain a LUT transformed signal; computing a chroma of the digital color signal from the digital color signal or the nonlinear transformed signal; computing a chroma weight coefficient based on the computed chroma; and performing arithmetic operations for weighting the nonlinear transformed signal and the LUT transformed signal with the chroma weight coefficient, thereby obtaining an output signal.
Preferably, the image processing method of the invention further includes the steps of: computing a lightness of the digital color signal from the digital color signal or the nonlinear transformed signal; computing a lightness weight coefficient based on the computed lightness; combining the lightness weight coefficient and the chroma weight coefficient to compute a single coordinated weight coefficient; and performing arithmetic operations for weighting the nonlinear transformed signal and the LUT transformed signal with the coordinated weight coefficient, thereby obtaining the output signal.
Preferably, in order to determine the coordinated weight coefficient, a sum or product of the two weight coefficients including the lightness weight coefficient and the chroma weight coefficient is computed and limiting is performed such that the coordinated weight coefficient takes a value within a specified range.
The nonlinear transformation is preferably a secondary matrix or a three-dimensional LUT.
The stated object of the invention can be attained by an image processing apparatus comprising: nonlinear transforming means for performing nonlinear transformation on a digital color signal; chroma computing means for computing a chroma of the digital color signal; chroma weight computing means for computing a chroma weight coefficient based on the chroma computed by the chroma computing means; and weighting arithmetic means for weighting a nonlinear transformed signal output from the nonlinear transforming means and the digital color signal with the chroma weight coefficient computed by the chroma weight computing means, thereby obtaining an output signal.
Preferably, the image processing apparatus of the invention further includes: lightness computing means for computing a lightness of the digital color signal; lightness weight computing means for computing a lightness weight coefficient based on the lightness computed by the lightness computing means; and weight coordinating means by which the chroma weight coefficient computed by the chroma weight computing means and the lightness weight coefficient computed by the lightness weight computing means are combined to compute a single coordinated weight coefficient, wherein the weighting arithmetic means performs arithmetic operations for weighting the nonlinear transformed signal output from the nonlinear transforming means and the digital color signal with the coordinated weight coefficient computed by the weight coordinating means.
Preferably, the image processing apparatus of the invention further includes: LUT transforming means for performing LUT transformation on the digital color signal, wherein the weighting arithmetic means performs arithmetic operations for weighting the nonlinear transformed signal output from the nonlinear transforming means and a LUT transformed signal output from the LUT transforming means with the chroma weight coefficient computed by the chroma weight computing means.
Preferably, the image processing apparatus of the invention further includes: lightness computing means for computing a lightness of the digital color signal; lightness weight computing means for computing a lightness weight coefficient based on the lightness computed by the lightness computing means; and weight coordinating means by which the chroma weight coefficient computed by the chroma weight computing means and the lightness weight coefficient computed by the lightness weight computing means are combined to compute a single coordinated weight coefficient, wherein the weighting arithmetic means performs arithmetic operations for weighting the nonlinear transformed signal output from the nonlinear transforming means and the LUT transformed signal output from the LUT transforming means with the coordinated weight coefficient computed by the weight coordinating means.
The nonlinear transforming means is preferably a secondary matrix or a three-dimensional LUT.
Preferably, the chroma computing means is selectively supplied with either the nonlinear transformed signal from the nonlinear transforming means or the digital color signal.
Preferably, the lightness computing means is selectively supplied with either the nonlinear transformed signal from the nonlinear transforming means or the digital color signal.
Preferably, the LUT transforming means is selectively supplied with either the nonlinear transformed signal from the nonlinear transforming means or the digital color signal.
Preferably, the weight coordinating means determines a sum of the two weight coefficients including the lightness weight coefficient and the chroma weight coefficient and performs limiting such that the coordinated weight coefficient takes a value within a specified range.
Preferably, the weight coordinating means determines a product of the two weight coefficients including the lightness weight coefficient and the chroma weight coefficient and performs limiting such that the coordinated weight coefficient takes a value within a specified range.